Multi-sample read operations can be performed in certain non-volatile memory devices. Consider the example of a resistive cross point array of non-volatile memory cells. A multi-sample read operation may be performed on a selected memory cell in the array by taking a first sample of the memory cell, and then taking one or more subsequent samples. The first sample generates a value corresponding to the logic value stored in the selected memory cell. The subsequent samples are used to generate a reference value. A comparison of the first-sampled value and the reference value indicates whether the first-sampled value corresponds to a logic ‘1’ or a logic ‘0.’
This multi-sample read operation is considered self-referencing because the first-sampled value is not compared to an external reference. Self-referencing read operations in general tend to be more reliable than read operations in which sensed values are compared to an external reference values. Moreover, due to limitations on the fabrication of certain cross point memory cell arrays, it can be difficult to find a single reference value for all of the memory cells in a large array.
Digital sense amplifiers can be used to perform multi-sample read operations on non-volatile memory. Consider the example of a digital sense amplifier including an integrator and a digital counter. A sense operation involves integrating a charge at a rate that depends upon the logic state of the selected memory cell, and determining the time for the charge to reach a threshold (the first sample may include one or more sense operations). The time is determined by using the digital counter to count clock pulses. A reference count is then subtracted from the clock pulse count (in a self-referencing operation, one or more samples are taken to generate the reference count). If CNT0<CNTR<CNT1, the most significant bit of the count indicates whether the logic value initially stored in the selected memory cell was a logic ‘1’ or a logic ‘0’ (CNTR is the reference count, CNT0 is the count corresponding to a logic 0, and CNT1 is the count corresponding to a logic 1).
Certain digital sense amplifiers output only the most significant bit of the count. The full contents of the digital counter are not made available.
During testing of the resistive cross point array, however, it can be helpful to know the contents of the digital counters, not just the sign of the most significant bit. The count at the end of the read operation can be used to determine signal-to-noise ratio (SNR). Measured as the ratio of the signal out of the digital amplifier to the noise generated within the digital amplifier where the signal is taken out, the SNR is a measure of reliability.
It would be desirable have the ability to determine the contents of the digital counter at the end of a multi-sample read operation. It would also be desirable to add this ability with a minimal amount of circuitry, since adding circuitry can increase the cost of the memory.